Method for manufacturing photomask blank

ABSTRACT

The present invention relates to a photomask blank obtained by forming a resist film after performing a silylation process on a silicon-containing inorganic film and provides a method for manufacturing a photomask blank having at least a silicon-containing inorganic film over a transparent substrate and a resist film on the inorganic film, comprising: forming the silicon-containing inorganic film; heat treating the formed silicon-containing inorganic film at a temperature more than 200° C. under an atmosphere containing oxygen; performing a silylation process after the heat treatment; and then forming the resist film by application. The method can inhibit generation of defects due to resist residues or the like after development.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a photomaskblank for a photomask for use in fabrication of a semiconductorintegrated circuit and the like.

2. Description of the Related Art

In recent semiconductor processes, particularly the increasing scale ofintegration of large-scale integrated circuits increasingly requires theshrinking of circuit patterns. There is a growing demand for shrinkingtechniques of wiring patterns formed in circuits or contact holepatterns for wiring between layers formed in cells. In production ofphotomasks, in which circuit patterns are formed, for use in opticalphotolithography for forming these wiring patterns or contact holepatterns, there is accordingly a need for a technique to precisely formfiner circuit patterns due to the above shrinking of the patterns.

Forming a resist pattern with high precision on a photomask blank isfirst needed to form a photomask pattern with higher precision on aphotomask substrate. When an actual semiconductor substrate isprocessed, since optical photolithography employs reduced-sizeprojection, the photomask pattern needs a size that is about four timesgreater than an actually needed pattern size. This however does not meanthat the required precision is accordingly lowered, but a higherprecision than that of the pattern after exposure is rather needed for aphotomask serving as an original.

In addition, photolithography currently employed draws a circuit patternwith a significantly lower size compared with a wavelength of light tobe used. Accordingly, if a photomask pattern having a size four timesgreater than that of a circuit form is used, then the photomask patternform is not accurately transferred to a resist film due to influence oflight interference occurring when the optical photolithography isperformed, etc. For the purpose of reducing the influence, the photomaskpattern sometimes needs to be formed into a more complex form (a form towhich optical proximity correction (the so-called OPC) is applied) thanan actual circuit pattern. There is accordingly a need for ahigher-precision processing method in the photolithography technique toobtain photomask patterns. The performance of lithography may berepresented by a resolution limit. The lithography technique inphotomask forming processes requires a resolution limit substantiallyidentical to or more than the resolution limit needed in opticalphotolithography employed in semiconductor processes using photomasks.

The procedure for forming a photomask pattern typically involves forminga photoresist film on a photomask blank having a light-shielding film ona transparent substrate, drawing a pattern by an electron beam,obtaining a resist pattern through development, and then etching thelight-shielding film to form a light-shielding pattern while using theobtained resist pattern as an etching mask. If it attempts to achievethe shrinking of the light-shielding pattern while the same thickness ofthe resist film as that before the shrinking is maintained, then a ratioof a film thickness to the pattern, i.e., the an aspect ratio, becomeslarger and pattern transfer thereby fails due to deterioration of thepattern form of the resist, or the resist pattern is broken or separatedin some cases. The shrinking accordingly necessitates thinning a resistfilm thickness.

Use of hard masks has been tried before to reduce a burden on resistsduring dry etching. For example, Patent Document 1 reports that an SiO₂film formed on MoSi₂ is used as an etching mask when MoSi₂ is dry etchedwith a gas containing chlorine, and the SiO₂ film can also function asan antireflection coating. In addition, there is disclosed that chromiumis used for a light-shielding film on a phase shift film and an SiO₂film on the light-shielding film is used as a hard mask in, for example,Patent Document 2.

PRIOR ART REFERENCES Patent Literature

-   Patent Document 1: Japanese Unexamined Patent publication (Kokai)    No. S63-85553-   Patent Document 2: Japanese Unexamined Patent publication (Kokai)    No. H7-49558

SUMMARY OF THE INVENTION Technical Problem

The above-described shrinking of patterns makes adhesion of resistsimportant. If a fine pattern with a size of, e.g., 50 nm or less isformed in a film containing Si on its surface above a photomask, thenthe resist pattern is separated due to poor adhesion to the filmcontaining Si during development. It has been known that performing asilylation process using, e.g., hexamethyldisilazane is effective inavoiding the separation.

There however arises a problem in that the silylation process giveshydrophobicity to a surface and thereby makes cleaning of the surfacedifficult; thereby many resist residues remain in a cleaning processafter development, resulting in defects. It is only necessary forimprovement in cleaning ability to improve wettability by usingisopropyl alcohol or the like. These solvents however adversely affectresist patterns.

The present invention was accomplished in view of the above-describedproblems and relates to a method for manufacturing a photomask blank,including forming a resist film after performing a silylation process ona silicon-containing inorganic film. It is an object of the presentinvention to provide a method for manufacturing a photomask blank thatcan inhibit generation of defects due to resist residues or the likeafter development.

Solution to Problem

To achieve this object, the present invention provides a method formanufacturing a photomask blank having at least a silicon-containinginorganic film over a transparent substrate and a resist film on theinorganic film, comprising: forming the silicon-containing inorganicfilm; heat treating the formed silicon-containing inorganic film at atemperature more than 200° C. under an atmosphere containing oxygen;performing a silylation process after the heat treatment; and thenforming the resist film by application.

In such a manner, the adhesion between the silicon-containing inorganicfilm and the resist film can be improved by the silylation process, andeven when a fine pattern is formed in the resist film, falling andseparation of the resist pattern can be inhibited.

In addition, since the resist film is formed after the silylationprocess following the heat treatment performed on the formedsilicon-containing inorganic film, generation of resist residues afterdevelopment, which are conventionally generated in the case ofperforming a silylation process, can be inhibited, and the number ofdefects can thereby be reduced. The method moreover is simple and easybecause the above heat treatment is merely performed before thesilylation process.

In the method, the heat treatment may be performed at a temperature morethan 400° C.

In such a manner, the generation of resist residues after developmentcan be further inhibited.

In the method, the silicon-containing inorganic film may further containoxygen.

Forming the silicon-containing inorganic film further containing oxygenas above is more preferable.

Moreover, the silicon-containing inorganic film may be composed ofsilicon and oxygen.

In this manner, the silicon-containing inorganic film is more preferablyformed from silicon and oxygen.

Moreover, a chromium-containing inorganic film may be formed beforeforming the silicon-containing inorganic film.

In photomask blanks, for example, a material containing chromium isoften used for light-shielding films. Forming a chromium-containinginorganic film enables the chromium-containing inorganic film to be usedas the light-shielding film or the like.

Moreover, a silicon-containing phase shift film may be formed on thetransparent substrate before forming the chromium-containing inorganicfilm.

In such a manner, a photomask blank including a phase shift film can bemanufactured and a photomask enabling a finer pattern to be formed witha higher resolution can be obtained.

Moreover, the silicon-containing inorganic film may be a hard mask.

In such a manner, the resist film can be thinned compared with the casewithout the hard mask, and a finer pattern (a pattern of 50 nm or less,for example) is readily formed.

Hexamethyldisilazane can be used for the silylation process.

Use of Hexamethyldisilazane (also referred to as HMDS, hereinafter) ispreferred because it is commonly used in processes of semiconductorfabrication, such as photomask blanks.

Advantageous Effects of Invention

As described above, the inventive method for manufacturing a photomaskblank enables the falling and separation of a resist pattern afterexposure and development to be inhibited. The method also enables thegeneration of resist residues to be inhibited, thereby reducing thenumber of defects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary photomask blankmanufactured by the inventive method;

FIG. 2 is a flowchart of an example of the inventive method formanufacturing a photomask blank;

FIG. 3 is an observation view showing defects distribution afterdevelopment in Example 1; and

FIG. 4 is an observation view showing defects distribution afterdevelopment in Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be specificallydescribed with reference to figures, but the present invention is notlimited to this embodiment.

The present inventors diligently studied photomask blanks. As describedabove, when a resist film is formed by application after asilicon-containing inorganic film is subjected to a silylation process,conventionally, many resist residues are generated after the resist filmis exposed and developed. The present inventors, in contrast, found thatperforming a heat treatment (at a temperature more than 200° C. under anoxygen-containing atmosphere) after forming the silicon-containinginorganic film enables defects such as resist residues to bedramatically reduced even when a resist pattern is formed through thefollowing silylation process, application of a resist, and development,thereby brought the present invention to completion.

FIG. 1 shows an exemplary photomask blank manufactured by the inventivemethod. As shown in FIG. 1, in the photomask blank 1, for example, asilicon-containing phase shift film 3, a chromium-containing inorganicfilm (e.g., a light-shielding film 4), a silicon-containing inorganicfilm 5, and a resist film 6 is formed in this order on a transparentsubstrate 2.

The transparent substrate 2 for use in the photomask blank 1 may becomposed of, but not limited to, a material that is transparent to anexposure light wavelength and has small amounts of thermal deformationat temperatures in manufacturing processes; a quartz substrate is givenas an example of this material.

Next, the structure of films over the transparent substrate 2 will bedescribed.

The silicon-containing inorganic film 5 may be an inorganic filmcontaining, for example, silicon; or silicon and at least one selectedfrom a group including oxygen, nitrogen, and carbon; or silicon andtransition metal; or silicon, transition metal, and at least oneselected from a group including oxygen, nitrogen, and carbon. Examplesof such an inorganic film include an inorganic film composed of silicon;or oxygen and silicon; or nitrogen and silicon; or oxygen, nitrogen andsilicon; or carbon and silicon; or carbon, oxygen and silicon; orcarbon, nitrogen and silicon; or carbon, oxygen, nitrogen and silicon.Examples of an inorganic film containing transition metal include aninorganic film composed of transition metal and silicon; or transitionmetal, silicon and oxygen; or transition metal, nitrogen and silicon; ortransition metal, oxygen, nitrogen and silicon; or transition metal,carbon, and silicon; or transition metal, carbon, oxygen, and silicon;or transition metal, carbon, nitrogen, and silicon; or transition metal,carbon, oxygen, nitrogen, and silicon.

An inorganic film composed of silicon, oxygen and nitrogen (an SiONfilm) and an inorganic film composed of silicon and oxygen (a SiO film)are particularly desirable.

Examples of the transition metal include molybdenum, tungsten, tantalum,titanium, zirconia, and hafnium. The silicon-containing inorganic film 5is not limited to a kind of a contained transition metal and may containtwo kinds or more of transition metals.

Hydrogen may also be contained.

The silicon-containing inorganic film 5 is subjected to the silylationprocess. The silicon-containing inorganic film 5, which is subjected tothe silylation process, thereby has high adhesion to the resist film 6formed thereon. The occurrence of falling and separation of a resistpattern can thereby be inhibited even when a fine pattern is formed inthe resist film 6.

The silylation process performed on the silicon-containing inorganicfilm 5 may be for example, but not limited to, a process using HMDS,which is commonly used in semiconductor fabrication processes.

The silicon-containing inorganic film 5 is subjected to the silylationprocess after being subjected to a heat treatment at a temperature morethan 200° C. under an oxygen-containing atmosphere.

In photomask blanks by a conventional method using a silylation process,many resist residues remain and cause defects after development. Incontrast, the inventive method for manufacturing the photomask blank 1can inhibit the generation of resist residues, which are generated inconventional photomask blanks, and reduce the number of defects.

The reason for the reduction in defects by the heat treatment isunclear; it can be understood that the reason is that the amount of OHor a bonding status of atoms in a contact plane between thesilicon-containing inorganic film 5 and the resist film 6 changes.

It is only necessary for the structure of films to locate thesilicon-containing inorganic film 5 directly under the resist film 6;the silicon-containing inorganic film 5 may function as, for example, anoptical film such as a light-shielding film or a phase shift film, or ahard mask film for use in forming a pattern in the optical film, andalso have an etching stopper film.

The present invention is particularly effective when a fine pattern,e.g., with a size of 50 nm or less, is formed. This effect is increasedwhen the silicon-containing inorganic film 5 is used as a hard maskfilm.

When the silicon-containing inorganic film 5 is used as a hard maskfilm, the film thickness of the hard mask film is preferably 1 to 30 nm,further preferably 1 to 20 nm, more preferably 1 to 10 nm.

In the configuration in this embodiment, a light-shielding film 4 and aphase shift film 3 below the light-shielding film 4 are formed betweenthe silicon-containing inorganic film 5 serving as the hard mask filmand the transparent substrate 2. The silicon-containing inorganic film 5serving as the hard mask film and the light-shielding film 4 preferablyhave etching selectiveness.

If the hard mask (etching mask) film (i.e., the silicon-containinginorganic film 5) is composed of a material that can be dry etched witha fluorine-based etching gas containing fluorine such as CF₄ or SF₆,then the etching process becomes easier; if the light-shielding film 4and the other inorganic films formed below the above hard mask film arecomposed of a material that has resistance against the fluorine-baseddry etching and can be etched by chlorine-based dry etching with anetching gas containing chlorine or chlorine and oxygen, then the etchingprocess becomes easier. The light-shielding film 4 and the otherinorganic films formed below the above hard mask film are desirablyinorganic films containing chromium: for example, a film containingchromium, or chromium and at least one selected from a group includingoxygen, nitrogen, and carbon. An antireflection layer may be formed onthe hard mask film side of the light-shielding film 4, and a filmcontaining a large amount of oxygen or nitrogen may be formed on itstransparent substrate side to improve the adhesion and to be used as anantireflection film.

When the phase shift film 3 is formed between the light-shielding film 4and the transparent substrate 2, the phase shift film 3 preferably hasdifferent etching properties from the light-shielding film 4. If thelight-shielding film 4 has resistance against the fluorine-based dryetching and can be dry etched with an etching gas containing chlorineand oxygen as above, the phase shift film 3 need only be composed of amaterial that has resistance against dry etching with an etching gascontaining chlorine and oxygen and can be etched by the fluorine-basedetching; for example, a material containing silicon and at least oneselected from a group including oxygen, nitrogen, and carbon may beused, or a transition metal may further be contained. Examples of thetransition metal include molybdenum, tungsten, tantalum, titanium,zirconia, and hafnium. Hydrogen may be further contained.

If a light-shielding film is located below the resist film 6, then thematerial containing silicon may be the above-described material and theentire light-shielding film may serve as the silicon-containinginorganic film 5. If an antireflection layer is formed on the surface ofa light-shielding film, then the antireflection layer (only) may serveas the silicon-containing inorganic film 5 and the light-shielding filmmay be formed of another material such as a chromium-containing film.

The material of the resist film 6 may be an electron beam resist for usein drawing with an electron beam or a photoresist for use in drawingwith light. A chemically amplified resist is particularly effective. Thechemically amplified resist may be either a positive type or a negativetype, and composed of a resin based on hydroxystyrene or mainly an acidgenerating agent; a cross-linking agent may be added; at least one ofquencher, surfactant and the like may be added; a (meth)acrylic acidresin may be used.

Next, the inventive method for manufacturing a photomask blank that canmanufacture the photomask blank 1 as shown in FIG. 1 will be described.FIG. 2 is a flowchart of an example of the inventive manufacturingmethod.

A transparent substrate 2 is first prepared (FIG. 2 at (A)). The abovetransparent substrate, for example a quartz substrate, may be preparedas the transparent substrate 2.

Next, a phase shift film 3 (FIG. 2 at (B)) containing silicon, i.e., theabove material, and a light-shielding film 4 (FIG. 2 at (C)) as achromium-containing inorganic film are formed in this order. The methodof forming these films may be, but not limited to, sputtering. The phaseshift film enables higher resolution.

Forming the phase shift film 3 and the light-shielding film 4 formedthereon, or forming the light-shielding film 4 and thesilicon-containing inorganic film 5 formed thereon with theabove-described materials each having different etching resistance makesthe etching process easier in production of photomasks.

Next, a silicon-containing inorganic film 5 composed of theabove-described material is formed (FIG. 2 at (Da)).

The method of forming the silicon-containing inorganic film 5 may bechemical vapor deposition (CVD) with a gas containing silicon, such as,for example, monosilane, dichlorosilane, or trichlorosilane; forming thefilm by sputtering using at least one target containing silicon ispreferable because this sputtering is easier and has goodcontrollability.

The method of forming the film by sputtering may be, but notparticularly limited to, DC sputtering, RF sputtering, or the like. Information of the silicon-containing inorganic film 5, when the inorganicfilm to be formed contains silicon and oxygen, for example, reactivesputtering may be performed with silicon as a target and a gas of argonand oxygen as a sputtering gas. When the inorganic film to be formedcontains nitrogen instead of oxygen, a nitrogen gas may be used insteadof the oxygen gas. When the inorganic film to be formed contains bothnitrogen and oxygen, a nitrogen gas and an oxygen gas may be used at thesame time, or a nitrogen oxide gas such as nitrogen monoxide or nitrogendioxide may be used. When the inorganic film to be formed furthercontains carbon, a gas containing carbon such as a methane gas, a carbonmonoxide gas, or a carbon dioxide gas may be used. When the inorganicfilm to be formed further contains transition metal, a target includingtransition metal and silicon may be used, or cosputtering with both asilicon target and a transition metal target may be performed.

In addition, the silicon-containing inorganic film formed as abovepreferably has the Si—Si bonds because the Si—O bonds on the surface ofthis film can be controlled by a heat treatment under anoxygen-containing gas.

Next, a heat treatment is performed (FIG. 2 at (Db)).

This heat treatment is performed at a temperature more than 200° C.under an atmosphere containing oxygen. The concentration of oxygencontained in the atmosphere in the heat treatment is not particularlylimited, and may be, for example, 1 to 100%. The heat treatment may be,but not limited to, infrared heating, resistance heating, or the like.

The temperature of the heat treatment need only be more than 200° C.,and is preferably more than 300° C., more preferably more than 400° C.,further preferably 450° C. or more; the temperature can be determinedproperly according to cost, the target number of defects afterdevelopment, and the like.

After the above heat treatment, a silylation process, which is describedlater, is performed, and a resist film is then formed by application.This procedure enables generation of resist residues to be inhibitedwhen a pattern is drawn on the resist film 6 and developed, as comparedwith conventional products, thereby significantly reducing the number ofdefects. Moreover, it is only necessary to perform the above heattreatment before the silylation process; the method is thus simple andeasy.

Cleaning is then performed to remove particles present on the surface ofa photomask blank (FIG. 2 at (E)). This cleaning may be performed byusing ultrapure water, or functional water, such as ultrapure watercontaining ozone, hydrogen, or the like, and applying ultrasonic waves.Alternatively, cleaning with ultrapure water including surfactant isfollowed by rinsing with ultrapure water, and the above functional watercleaning, ultraviolet light irradiation, or the combination thereof isperformed.

The silylation process is then performed to reduce surface energy of aphotomask blank surface such that the photomask blank surface isalkylsilylated (FIG. 2 at (F)). Such a silylation process allows a fineresist pattern to be prevented from being separated and fallen.

The above HMDS is given as an example of a silylation agent, but doesnot limit the silylation agent.

Known silylation processes include a method of directly applying thesilylation agent to the silicon-containing inorganic film of thesubstrate and a method of exposing the substrate to the silylationagent. Known exposing methods include a method of evaporating thesilylation agent in a container holding the substrate and a method ofvaporizing the silylation agent by bubbling of a nitrogen gas. Thetemperature at which the silylation agent is reacted may be, forexample, in the range from 40° C. to 200° C. The process time, forexample, is preferably adjusted such that the wettability of thesubstrate becomes a proper value by previously measuring a contact angleof water under the same conditions as the silylation processes.

The above-described resist film 6 is then formed on thesilicon-containing inorganic film 5 by application after the silylationprocess, whereby a photomask blank 1 can be obtained (FIG. 2 at (G)).

The applying method is not particularly limited and may be performed,for example, in the same manner as a conventional method. The thicknessof the resist film may be appropriately determined to obtain a goodpattern form.

EXAMPLE

The present invention will be specifically described below throughexamples and comparative example, but the present invention is notlimited these examples.

Example 1

A photomask blank was manufactured by the inventive manufacturingmethod.

A phase shift film of MoSiON with a thickness of 75 nm was formed on 152mm squares of a quartz substrate having a thickness of about 6 mm bysputtering. A gas of oxygen, nitrogen, and argon was used as asputtering gas; MoSi₂ and Si were used as two types of target. Thesubstrate was rotated at 30 rpm to form the film.

The composition of the phase shift film was Mo:Si:O:N=1:4:1:4 (atomicratio), which was investigated by an electron spectroscopy for chemicalanalysis (ESCA) method (also referred to as an XPS method) with K-Alpha(manufactured by Thermo Fisher Scientific K.K.).

On the phase shift film, a CrN layer (30 nm) and a CrON layer (20 nm) aslight-shielding films were formed in order from a substrate side upwardby sputtering. A gas of argon and nitrogen was used as a sputtering gasfor the CrN layer; a gas of oxygen, nitrogen, and argon was used as asputtering gas for the CrON layer; a chrome metal was used as a target.The substrate was rotated at 30 rpm to form the film.

The compositions of the light-shielding films were Cr:N=9:1 (atomicratio) for the CrN layer and Cr:O:N=4:5:1 (atomic ratio) for the CrONlayer, which were investigated by the ESCA.

On the top of the light-shielding films, an SiO film with a thickness of5 nm as an etching mask film (a hard mask) containing silicon was formedby sputtering. A gas of oxygen and argon was used as a sputtering gas;Si was used as a target. The substrate was rotated at 30 rpm to form thefilm.

The investigation of the SiO by the ESCA revealed that the Si—Si bondswere seen.

The resultant was heated at 500° C. under an atmosphere of 20% of oxygenand 80% of nitrogen, and the silylation process was then performed withHMDS. After that, a negative type of electron beam resist (manufacturedby Shin-Etsu Chemical Co., Ltd.,) was applied and developed withtetramethylammonium hydroxide. The resultant was rinsed with pure water.

The investigation of the resultant with a defect inspection apparatusMAGICS 2350 (manufactured by Lasertec Corporation) demonstrated a goodresult in which, as shown in FIG. 3, the number of defects was extremelysmall. The number of detected defects having a size of 0.1 μm or morewas 42.

Example 2

A photomask blank was obtained as in Example 1 except that thetemperature of the heat treatment was changed into 300° C.

The number of defects after development was consequently 1180.

Comparative Example 1

A photomask blank was manufactured as in Example 1 except that the heattreatment was not performed.

The investigation of defects after development revealed that, as shownin FIG. 4, many resist residues remained.

The number of detected defects having a size of 0.1 μm or more was 4704,which was very large.

It is to be noted that the present invention is not limited to theforegoing embodiment. The embodiment is just an exemplification, and anyexamples that have substantially the same feature and demonstrate thesame functions and effects as those in the technical concept describedin claims of the present invention are included in the technical scopeof the present invention.

What is claimed is:
 1. A method for manufacturing a photomask blankhaving at least a silicon-containing inorganic film over a transparentsubstrate and a resist film on the inorganic film, comprising: formingthe silicon-containing inorganic film; heat treating the formedsilicon-containing inorganic film at a temperature more than 200° C.under an atmosphere containing oxygen; performing a silylation processafter the heat treatment; and then forming the resist film byapplication.
 2. The method for manufacturing a photomask blank accordingto claim 1, wherein the heat treatment is performed at a temperaturemore than 400° C.
 3. The method for manufacturing a photomask blankaccording to claim 1, wherein the silicon-containing inorganic filmfurther contains oxygen.
 4. The method for manufacturing a photomaskblank according to claim 2, wherein the silicon-containing inorganicfilm further contains oxygen.
 5. The method for manufacturing aphotomask blank according to claim 3, wherein the silicon-containinginorganic film is composed of silicon and oxygen.
 6. The method formanufacturing a photomask blank according to claim 4, wherein thesilicon-containing inorganic film is composed of silicon and oxygen. 7.The method for manufacturing a photomask blank according to claim 1,wherein a chromium-containing inorganic film is formed before formingthe silicon-containing inorganic film.
 8. The method for manufacturing aphotomask blank according to claim 2, wherein a chromium-containinginorganic film is formed before forming the silicon-containing inorganicfilm.
 9. The method for manufacturing a photomask blank according toclaim 3, wherein a chromium-containing inorganic film is formed beforeforming the silicon-containing inorganic film.
 10. The method formanufacturing a photomask blank according to claim 4, wherein achromium-containing inorganic film is formed before forming thesilicon-containing inorganic film.
 11. The method for manufacturing aphotomask blank according to claim 5, wherein a chromium-containinginorganic film is formed before forming the silicon-containing inorganicfilm.
 12. The method for manufacturing a photomask blank according toclaim 6, wherein a chromium-containing inorganic film is formed beforeforming the silicon-containing inorganic film.
 13. The method formanufacturing a photomask blank according to claim 7, wherein asilicon-containing phase shift film is formed on the transparentsubstrate before forming the chromium-containing inorganic film.
 14. Themethod for manufacturing a photomask blank according to claim 8, whereina silicon-containing phase shift film is formed on the transparentsubstrate before forming the chromium-containing inorganic film.
 15. Themethod for manufacturing a photomask blank according to claim 9, whereina silicon-containing phase shift film is formed on the transparentsubstrate before forming the chromium-containing inorganic film.
 16. Themethod for manufacturing a photomask blank according to claim 10,wherein a silicon-containing phase shift film is formed on thetransparent substrate before forming the chromium-containing inorganicfilm.
 17. The method for manufacturing a photomask blank according toclaim 11, wherein a silicon-containing phase shift film is formed on thetransparent substrate before forming the chromium-containing inorganicfilm.
 18. The method for manufacturing a photomask blank according toclaim 12, wherein a silicon-containing phase shift film is formed on thetransparent substrate before forming the chromium-containing inorganicfilm.
 19. The method for manufacturing a photomask blank according toclaim 1, wherein the silicon-containing inorganic film is a hard mask.20. The method for manufacturing a photomask blank according to claim18, wherein the silicon-containing inorganic film is a hard mask. 21.The method for manufacturing a photomask blank according to claim 1,wherein hexamethyldisilazane is used for the silylation process.
 22. Themethod for manufacturing a photomask blank according to claim 20,wherein hexamethyldisilazane is used for the silylation process.